Endocrine SystemAdrenal. Thyroid. Parathyroid.

Adrenal GlandsPaired organs that cap the superior borders of the

kidneysEach consists of:

Adrenal cortex (outer)– derived from neural crest ectoderm– does not receive neural innervation so it must be

stimulated hormonally- Consists of three zones: zona glomerulosa, zona

fasciculata and zona reticularisAdrenal medulla (inner)– derived from mesoderm– innervated by preganglionic sympathetic nerve fibers

Structure of Adrenal Gland

Functions of Adrenal CortexSecretes steroid hormones (corticosteroids

or corticoids)Mineralocorticoids – regulate Na+ and K+

balanceGlucocorticoids – regulate the metabolism of

glucose and other organic moleculesSex steroids – weak androgens

(dehydroepiandrosterone or DHEA)

AldosteroneMost potent mineralocorticoidSecreted by the zona glomerulosaStimulate the kidneys to retain NaCl and

water while excreting K+ in the urineThese actions help to increase the blood

volume and pressure and to regulate blood electrolyte balance

CortisolPredominant glucocorticoidSecreted by zona fasciculata and perhaps

also the zona reticularisSecretion is stimulated by the ACTH from

the anterior pituitary glandhave effects on metabolism:

Stimulate gluconeogenesisInhibit glucose utilizationPromote lipolysis and consequent release of

free fatty acids into the blood

Exogenous GlucocorticoidsTaken as pills, injections, sprays and topical

creamsUsed medically to suppress the immune

responseVery useful in treating inflammatory diseases

(asthma, rheumatoid arthritis)Side effects:

HyperglycemiaDecreased glucose toleranceDecrease synthesis of collagen and other

extracellular matrix proteins and increased bone resorption (osteoporosis)

Cushing’s SyndromeHypersecretion of ACTH (glucocorticoid)Can also result from a tumor of the adrenal

cortexCharacterized by:

Changes in carbohydrate and protein metabolism

HyperglycemiaHypertensionMuscular weakness

Functions of Adrenal MedullaSecrete epinephrine and norepinephrineEffects of catecholamine hormones secreted by the

medulla:Increase cardiac outputDilate coronary blood vesselsIncrease mental alertnessIncrease respiratory rateElevates metabolic rate

Supported by the metabolic actions of E and NE:Rise in blood glucose (due to stimulation of hepatic

glycogenolysis)Rise in blood fatty acids (due to lipolysis)

Stress and the Adrenal Gland1936 – Hans Selye discovered that injections

of cattle ovary extract into rats:1. stimulated growth of the adrenal cortex2. caused the atrophy of the lymphoid tissue of the spleen, lymph nodes and thymus, and3. produced bleeding peptic ulcers

Same things happened when he subjected the rats to cold environment and when he dropped them into water and made them swim until they were exhausted

Stress and the Adrenal GlandStress – reaction of the organism to stimuli

(stressors) which may produce damaging effects

Stressors stimulate the pituitary-adrenal axisUnder stressful conditions, there is a

increased secretion of ACTH therefore there is the increased secretion of glucocorticoids

Stress and the Adrenal GlandThere is a nonspecific response of the body to

readjust itself following any demand made upon itA rise in the plasma glucocorticoid level results

from the demands of the stressors (General Adaptation Syndrome or GAS)

3 stages in response to stress:Alarm reaction (adrenal glands are activated)Stage of resistance (readjustment occurs)Stage of exhaustion (if readjustment is not

complete – this leads to sickness and possibly, death)

Thyroid GlandLocated just below the larynxTwo lobes are positioned on either side of

the trachea and are connected anteriorly by a medial mass of the thyroid tissue (isthmus)

Largest pure endocrine gland (20-25 grams in wt.)

Thyroid Gland

Thyroid GlandConsists of numerous spherical hollow sacs

(thyroid follicles) lined with simple cuboidal epithelium composed of follicular cells (secrete thyroxin)

The interior of follicles contain colloid (protein-rich fluid)

Also contains parafollicular cells (secrete calcitonin or thyrocalcitonin)

Thyroid Gland

Production and Action of Thyroid HormonesThyroid follicles actively accumulate iodide (I-)

from the blood and secrete it into the colloidIodide will be oxidized to form iodine once it

enters the colloid then it will be attached to amino acids (tyrosines) within the polypeptide chain of protein (called thyroglobulin)

The attachment of:One iodine to tyrosine => monoiodotyrosine

(MIT)Two iodines => diiodotyrosine (DIT)

Production and Action of Thyroid HormonesWithin the colloid, enzymes modify the structure

of MIT and DIT and couple them togetherTetraiodothyronine (T4) or thyroxine –

produced when two DIT molecules that are appropriately modified are coupled together

Triiodothyronine (T3) – combination of one MIT with one DIT

Upon stimulation of TSH, the cells of the follicle take up a small volume of colloid by pinocytosis, hydrolyze the T3 and T4 from the thyroglobulin, and secrete free hormones into the blood

Production and Action of Thyroid HormonesThrough the activation of genes, thyroid hormones:

Stimulate protein synthesisPromote maturation of the nervous systemIncrease the rate of cell respiration

Through this, thyroxin elevates the basal metabolic rate (the resting state of calorie expenditure by the body)

Calcitonin– works with parathyroid hormones to regulate

calcium levels of the blood- Inhibits dissolution of the calcium phosphate crystals

of bone and stimulates the excretion of calcium in the urine by the kidneys

HyperthyroidismIodine-deficiency (endemic) Goiter– abnormal growth of the thyroid gland– due to the absence of the dietary iodine, the

thyroid cannot produce adequate amounts of T4 and T3

– the resulting lack of negative feedback inhibition causes hypersecretion of TSH that stimulates abnormality

HypothyroidismCondition of people who have inadequate

secretion of the thyroid hormonePeople who have hypothyroidism have an

abnormally low basal metabolic rate and they experience weight gain and lethargy

In adults, it causes:Myxedema– accumulation of mucoproteins and fluid in

subcutaneous connective tissues– symptoms: swelling of the hands, face, feet and

tissues around the eyes

HypothyroidismCan also be caused by insufficient

thyrotropin-releasing hormone (TRH) secretion from the hypothalamus, or insufficient secretion of TSH from the anterior pituitary or insufficient iodine in the diet

Parathyroid GlandsSmall flattened glands that are embedded in

the posterior surfaces of the lateral lobes of the thyroid gland

Parathyroid Glands

Parathyroid hormoneOnly hormone secreted by the parathyroid

glandsImportant in the control of calcium levels of

the bloodPromotes rise in blood calcium levels by

acting on the bones, kidneys and intestine